1. Field of the Invention
This invention relates generally to drill bits and more particularly to gun drill bits suitable for creating a deep hole in a solid material.
2. Prior Art
Generally, for creating a deep hole in a solid material, there are employed drill bits of the type in which the bit body has a V-shaped straight groove extending longitudinally along the side thereof. One of the side faces defining the longitudinal groove terminates at its forward end in a cutting edge, and the bit body has a drilling fluid passageway extending longitudinally therethrough. A typical example of such drill bits is a gun drill bit.
FIGS. 1 and 2 show the forward end or head portion of one conventional gun drill bit, the drill bit comprising a cylindrical body 100 having a straight V-shaped groove 101 extending longitudinally along the side thereof. The bit body has a drilling fluid passageway 102 extending longitudinally therethrough. The longitudinal groove 101 serves to discharge cuttings from a drilled hole in a workpiece. The groove 101 is defined by a pair of radially disposed flat side faces 103, 104, the side face 103 acting as a rake surface and terminating at its forward end in a pair of continuous straight inward and outward cutting edges 105, 106. The cutting edges 105, 106 intersect each other at the apex 107 of the bit head and are sloping in a direction away from the bit head, the apex being offset one sixth to one third of the diameter of the cylindrical bit body from the central axis thereof. Thus, the inward and outward cutting edges 105, 106 are sloping in the opposite directions. As a result, during the drilling operation, the inward and outward cutting edges 105 and 106 produce one-piece cuttings, each cutting having alternate portions produced by the inward and outward cutting edges 105 and 106. Each cutting is deformed at the junctions of the alternate portions so that the alternate portions interfere with each other to thereby allow the cutting to be susceptible to division at the junctions of the alternate portions. This cutting is divided by impinging on the side face 104 or being subjected to the high pressure of the drilling fluid passing through the longitudinal groove 101. The thus divided cuttings are relatively long. The smaller the cuttings are divided, the more efficiently the drilling operation is carried out because the cuttings do not interfere with the cutting operation of the drill bit. This conventional drill bit has been found not entirely satisfactory in that the cuttings produced by the inward and outward cutting edges 105, 106 fail to be divided in pieces enough to permit the cutting operation to be carried out in an efficient manner. Usually, the maximum feed rate or cutting speed of this known drill bit is only 0.05 mm per revolution.